This invention relates generally to semiconductor wafer supports, and more particularly to a low thermal mass semiconductor wafer support for use in heat treatment of semiconductor wafers in a furnace.
High temperature heat-treatment (e.g., annealing) of semiconductor wafers is commonly used to achieve certain desirable characteristics. For example, such a process may be used to create a defect free layer of silicon on the wafers. The high temperature annealing process of the type to which the present invention particularly relates is carried out in a vertical furnace which typically subjects the wafers to temperatures above 1100 degrees C., e.g., between about 1200 degrees C. and about 1300 degrees C.
During high temperature heat-treatment, at temperatures above 750 degrees C. and especially above 1100 degrees C., the silicon wafers become more plastic. If the silicon wafers are not adequately supported during heat treatment, the wafers may undergo slip due to local gravitational and thermal stresses. As is well known in the art, slip may introduce contaminants into the device areas of the wafers. Moreover, excessive slip may lead to plastic deformation of the wafers, which in turn may lead to production problems, such as photolithography overlay failures causing yield losses in device manufacture.
Vertical wafer boats or racks are used to adequately support semiconductor wafers, and, to minimize the local gravitational and thermal stresses. The support attempts to prevent slip and plastic deformation while the wafers are being heat treated. A typical vertical wafer boat used in a vertical furnace comprises three or more vertical rails also referred to as rods. The rods typically have grooves or laterally extending fingers for supporting the wafers between the vertical rods within the boat. Each wafer may rest directly on fingers (or grooves) lying generally in a common horizontal plane. This configuration is common in the older art and is adequate when heat treating 200 mm and smaller diameter wafers. Alternatively, each wafer may rest on a wafer holder platform, e.g., a ring or solid plate, supported by the fingers (or in grooves), lying generally in a common horizontal plane. This configuration is common in the newer art and is usually necessary to adequately support 300 mm and larger diameter wafers. The 300 mm and larger diameter wafers are subjected to more local gravitational and thermal stresses than smaller diameter wafers, and the wafer holder platforms better support the 300 mm wafers by increasing the areas of the wafers that are supported.
Co-assigned U.S. Pat. No. 7,033,168, which is incorporated herein by reference, shows one suitable type of wafer boat for use in the above-described process. WO 2006/118774 ('774) shows another type that includes shelves for supporting the wafers. Each shelf 24 includes a plurality of holes 32, 34 that “are useful to relieve stress in the thin shelves 24, to reduce thermal mass, and to allow the furnace ambient to reach a substantial portion of the wafer backside, thereby preventing the wafers from sticking to the shelves 24. The planar shape of the shelf 24 allows it to be economically machined from slices of material, especially silicon, similar to wafers.”
However, there are certain shortcomings in the prior art supports. For example, placing the wafers in contact with the hole edges of the '774 shelf will cause contamination of the wafer. Additionally, the '774 configuration will cause slip in the wafer. A better wafer support that will not cause contamination or slip is needed.